Wireless power transfer for implantable medical devices

For the new generation of Active Implantable Medical Devices (AIMDs), technological advancement in electronic and micro and nanofabrication is going in the direction of reduction of size and power consumption, and improvement of therapeutic efficacy and safety. The power supply of implantable devices is still an open challenge. Most AIMDs are powered by a primary battery that takes up much space, and that has to be replaced when running out. Also, one of the main challenges is to ensure proper and sufficient power delivery to the entire device without overheating it. We have developed a Wireless Power Transfer (WPT) link using the Near-field Resonant Inductive Coupling (NRIC) to recharge subcutaneously implanted batteries and to power supply AIMDs without a battery. We have evaluated key performance parameters for the WPT system, taking into account size, maximum power dissipation, and thermal performance of the receiver, energy provisioning from the transmitter, and overall efficiency. First, we have characterized the WPT link by testing the transmission efficiency with respect to both the distance of the coils (up to 20 mm) and their alignment and by varying the load (simulated battery). Tests were carried out with interposed air and then with a conductive environment with high permittivity (animal tissue) to simulate skin behavior. Another essential aspect that has been taken into account is the heating of the device, which must be kept under control to avoid overheating the surrounding tissue. We have then developed a hardware and firmware communication system between the receiver and transmitter to improve the power transfer efficiency (PTE). Measurements show that the maximum power efficiency achievable goes from 26.7% to 45.4% for 10 mA to 100 mA charging current at a relative distance of coils from 2.8 to 10 mm. The power transmission is not affected by the interposition of animal tissue. We have performed the recharge of three different batteries at 25 mA and 10 mA with a relative coils’ distance of 10 mm to validate the system. The temperature increase above room temperature of the Tx coil and the Rx receiver is no more than 4 °C and 2 °C, respectively, during the 10 mA recharge phase.